Is this Mullard EL34 good? How to visually ID "spent" tubes?

Discussion in 'Amps and Cabs' started by EL34, Jan 24, 2006.

  1. EL34

    EL34 Silver Supporting Member

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    I am by no means an expert on old tubes, but I recently picked up a quad of used Mullards that were said to have plenty of life left in them. The flashing is not bright/shiny on any of them, it's rather a dull grey color. But one of the four tubes is pictured below. To me, it looks like this tube is toast, and I would be loathe to run it in my amp for any period of time for fear of it giving up the ghost. There is no flashing left, and it just appears to be burned in the area where the flashing used to be.

    Is this tube good, or should I be asking for my money back?

    [​IMG]

    [​IMG]
     
  2. dewman

    dewman Gold Supporting Member

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    looks bad, I'm not an expert (email Mike K at kcanostubes.com for his opinion) but to me they look like they might be cooked. they should be tested at operating voltages, not on some low voltage tube tester. Its not a meaningful comparison, as many people do selling tubes on ebay. I would get your money back, then call Mike at KCANOS tubes http://www.kcanostubes.com/content/, or Lord Valve http://www.nebsnow.com/LordValve , or other reputable dealers (usually they advertise constantly in Vintage Guitar magazine). Its true they might have some life, but that could be days and not the months that you want...and the tone could be suspect too, which, after all , is why you bought Mullards anyways...
     
  3. stratovarius

    stratovarius Supporting Member

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    Ahhh eBay! Three out of four seems to be about par for me! I've gotten a couple of Mullards that look like that and they both turned out to be bad. I worry about "experimenting" with my amps, too.
     
  4. Blue Strat

    Blue Strat Member

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    From appearance, I'd have to guess you've got about 5% life, if any, left in that tube. I'll be it tests nowhere near "new".
     
  5. EL34

    EL34 Silver Supporting Member

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    Thanks for the feedback. Actually, I did not purchase them on ebay, I purchased them here on TGP. You really need to be careful buying used gear any more. I am close to swearing off any more used gear purchases.
     
  6. highgeer

    highgeer Member

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    EL34,
    Have you put the tubes in an amp? Many of the used NOS tubes that I have had over the years looked awful, but they sounded great and lasted a long time(I do not know anything about nos tubes except how they sound). What did you pay for them? I think that a true NOS quad would run over $700. If you paid $700 you got screwed bigtime. Of course a guy selling true NOS tubes(for north of $700) is going to tell you that these tubes suck, and that you should always buy new ones. I am not sure how someone can judge a tube by it's photo, even if you are an expert. I have used tubes that look worse than this one with great performance over several years. I think you should put them in an amp before you jump to conclusions. Most of the guys on this forum are not out to rip you off(espeically the members that have been around for a awhile). I am always leary of buying and selling anything and that is why I refuse to sell or buy something from anyone I have not talked to on the phone. I have bought and sold an insane amount of gear on this forum, and 99% of the deals have been great.

    Just my 2 cents,
    Mike
     
  7. fullerplast

    fullerplast Senior Member

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    +1

    The flashing is just one thing to look at but doesn't tell the whole story. It would be more useful if you could test them for gm in a tester, and even more useful if you could reduce the filament voltage by 10% and see how the gm hung in there. Anything below 75% or so of new gm value is getting into the replacement zone. If the gm drops by more than 25% with reduced filament voltage, same thing-not a great deal of life left.

    Also consider this: many of these great old tubes were made with 5000-10,000 hours of operational life in mind. Of course this depends greatly on operating conditions, but even at 5000 hours, if you have 20% life left, that's 1000 hours....which translates into 250 four hour gigs....which translates into a year of gigging 5 days a week!
     
  8. Blue Strat

    Blue Strat Member

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    How about putting them in a tester before putting them in an amp (or is that a big capitalist conspiracy too? ;) )
     
  9. outtahear

    outtahear Supporting Member

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    Lotta good points made by all. Here's another- Please-If you are going to sell older tubes (or used newer stock,) there is really no reason to not post pictures. :AOK Or at dead least, have them available for e-mailing. Raise your right hand and repeat after me...:D
     
  10. EL34

    EL34 Silver Supporting Member

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    Thanks for everyones input. There are other issues here as well...the tube with the burnt getter also has severe discoloration down one side of the plates. Also, another one of the tubes has the mica spacer substantially delaminating. I am just not comfortable putting these tubes in my Komet, no way no how. If I had been sent pics showing all angles of the tubes, then I never would have bought them....lesson learned.

    I've contacted the seller, and he says he is considering a refund.
     
  11. Roccaforte Amps

    Roccaforte Amps Member

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    I agree, I won't use anything that looks like that,
    I don't care how well it tests.
    The getter has a very important roll in tube life,
    and that getter is spent.
    I'd love to see the gas tests for this tube!
     
  12. sws1

    sws1 Member

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    How about this rule?

    Don't call it "NEW OLD STOCK (NOS) IF IT AIN'T NEW". It's USED!
     
  13. Tele71

    Tele71 Gold Supporting Member

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    I personally don't see the reason to risk damage to your amp using "NOS" tubes when you can spend the same amount of $ on JJs, TAD, Winged C, or some of the other brands of tubes on the market these days. True they may not last as long but their price vs. NOS tubes allows for replacement twice a year. I you had to replace 700$ worth of "NOS" tubes twice a year that could be costly. I have bought many NOS tubes and used them in my black face amps and none have sounded better then the jjs i have in my twin. I had an amp built for me this year and when it came time to tube her i went with new manufacture tubes because you never really know what a tube has been through in it's life time. And one thing you CAN be certain of is the manufacture of tubes from companies like RCA, Mullard, Tung sol, Phillips and the like doesn't happen anymore couple that with demand and there just aren't enough NOS tubes to go around. So Prices are high and quality is questionable at best. Besides all those audiophile guys are hoarding all good NOS stuff anyways YMMV. MHO
    Paul
     
  14. Blue Strat

    Blue Strat Member

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    The risk of using REAL NOS tubes is less or equal to using new "third world" tubes. Using the tube pictured above ("NOS") could pose a serious risk to your $4000 amp. But then, I sell tubes for a living, why trust me? ;)
     
  15. highgeer

    highgeer Member

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    Mike,
    Please forgive my ignorance. What risk could these tubes pose. I have used numerous discolored tubes in all the Komet that I have owned, as well as countless other amps. A fuse always blew before anything bad happened to the amp. I have also been shipped brand new amps straight from the builder that had bad tubes(including NOS), and again, a fuse blew before something bad happened. I did have to replace a biasing resistor once, but we never desided if it was the tube or a bad resistor. I respect your opinion and now Doug's, but I still think it is difficult to make a judgement from a few pics. BTW, I'm definitely a capitalist, I make custom furniture for a living. If I need any NOS tube you would definitely be one of the guys I would call.

    EL34,
    Again, how much did you pay for them? Did the amount reflect the condition? Did the guy test them? Misrepresent them? If I was that worried about my amp then i would have never bought used tubes. I would have bit the bullet and bought a pair from Mike or Gregg, or even better had Gene and Hogy put a set in there.

    Mike
     
  16. fullerplast

    fullerplast Senior Member

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    +1

    If you run an amp long enough a tube will eventually fail. It's just a fact of life. Any good amp will be designed such that a tube failure will not hurt the amp. Tubes rarely will fail in a catastrophic way unless the ratings are exceeded and that can happen when they are new. More commonly, it is just a gradual deterioration of gain and/or tone. When it gets to the point of it being noticeable (as in "my amp doesn't seem to sound as good as I remember it sounding"), then new tubes will bring it back to life.

    A UOS (used old stock) tube can often sound better than new production tubes. That's a fact, but there are no absolute rules. Some new production may sound better than some NOS or UOS. A UOS tube can still last longer than a new production tube. That's also a fact. You don't have to look any further than this forum to read reports of all the early failures of new tubes.

    People buy UOS tubes because they can often get better tone than new production at a cheaper cost than NOS. It's a tradeoff. If you have the bucks, you can always buy NOS or new production-whatever you prefer. If you don't have the bucks, then you can get some great tone using UOS with life still left in them. Just like some folks get new cars every two years, some folks change tubes way before they wear out. You can get some good deals that way. Like cars, the most depreciation comes in the first third of the life of the tube. There can be plenty of useful life left, especially for people using an amp for only a few hours a week.

    You just need to use common sense. There are several ways to check a tube before plugging it into an amp. If the visual and tester checks all pass, you need to try it at real voltages. You can even use a variac starting at 95V if you are paranoid. Turn the amp on and look at the tubes for any unusual signs of a problem-like glowing plates. Bias the tubes properly. If all looks good, run the amp for a while. Make sure nothing changes over a period of hours. Do this with new production tubes also. Sooner or later a tube will fail in your amp. Again, if the amp is designed properly this will not be a catastrophic event. Most often, it will be simply diminished tone or output.
     
  17. Blue Strat

    Blue Strat Member

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    The risk is in not testing them first. The fact that they've obviously got lots of hours on them makes them higher risk, in general, than tubes that have fewer hours and have been tested.

    I've seen 100s of amps with smoked screen grid resistors (not a big deal, but still requires work by an amp tech if you can't replace it yourself) caused by shorted tubes that weren't caught by fuses. I've also seen amps in which tubes running at 4X maximum dissipation (due to bias failure) kept plugging along without the fuse blowing. In some cases, the OT overheated, opened, and required replacement, and in others it was just a matter of time before the OT would self destruct.

    Feel free to poo poo 10 years of experience with tube amps, a BSEE degree and 25 years of corporate engineering work, and common sense. I wouldn't take those chances with my own $4000 amp (if I had one) knowing that I could repair it at virtually no cost.
     
  18. fullerplast

    fullerplast Senior Member

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    Outside of simply wearing out, tubes can fail by shorting or by going gassy. The shorting is due to elements touching internally, usually because the tubes were run too hot and the elements warped or were damaged. This can happen to new or old tubes and is easily tested in most any tester.

    Gas is just air in the tube, and it will make the tube run hotter in general and make it easier to cause shorting if it gets hot enough. This is also easily tested.

    Just because the getter is brownish, it doesn't mean that the tube will be more likely to have a short. It means it's definately seen some use, but until you test the tube it's hard to draw conclusions based on that alone, other than it's had some hours on it. It still may test better and last longer than some new production tubes.

    Buying used tubes is always a cost/performance tradeoff. I'd certainly trust some used tubes more than some new production tubes, and vice versa. You can get some great performing tubes at a fraction of new cost if you choose carefully. Risk is minimal IMHO if you test for shorts and gas prior to checkout in an amp. The worst scenario for a tube failure is a short, and that can happen anytime a tube is run at or near it's limits, new or used.

    Not poo-pooing any of Mikes experience, but he's not the only one with some: 30+ years experience with tube amps, Johns Hopkins MSEE, 30 years corporate aerospace circuit design and system engineering, maybe even a bit of common sense too:BEER .
     

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